Presently, semiconductor based image sensors are widely used, especially in digital cameras. These cameras exist as both stand-alone devices and as integrated in other multipurpose electronic devices such as cell phones or personal digital assistants (PDAs).
The marketplace continues to drive for improvements in image capture. These improvements include not only the quality of the captured images, such as improved color accuracy and resolution, but also the lowest possible cost and smallest size.
A common method to capture color information in cameras using semiconductor based image sensors is to employee a mosaic, such as a Bayer pattern, of alternating red, green, and blue pixels. The light reaching these pixels is filtered by corresponding red, green, or blue light filter films made out of materials such as polyimide. However, this method causes several problems. First, conventional RGB filters significantly attenuate the light. It is common for the green and red filters to have less than 40% transmittance and the blue filter to have less than 25% transmittance. This reduction in transmitted light leads to a major decrease in the sensitivity of the sensor array. Second, the film filters commonly used have considerable spectrum overlap. For example, the red filter allows some green light to pass, the green filter allows some red and blue light to pass, and the blue filter allows some green light and red light to pass. This makes determination of the true colors of an image very difficult. Third, the general use of a mosaic of pixels, such as a Bayer pattern, to determine color, degrades the potential spatial resolution of a given array. The amount of degradation depends on lighting conditions but can approach 75% for high resolution systems. For example, a lens with a module transfer function (MTF) value of 0.7 at 100 lp/mm may only provide an MTF of 0.7 at 25 lp/mm for a module with a Bayer mosaic semiconductor image sensor.
Another known method for capturing color information involves Foveon sensors. With the Foveon design, color filtering is achieved by selective absorption of the light through the sensor semiconductor substrate. Like the systems using color films, the Foveon type system still suffers from considerable color spectrum overlap between the different color sensors, thus making the determination of true color by the system difficult. For example, the signal from a blue sensor will have about 35% green light contribution and 10% red light contribution under sunlight conditions. This problem is further aggravated by the fact that the contribution from each color will change under different lighting conditions such as office light vs. sunlight.
Another known method for color filtering that could be applied to imagers is disclosed in U.S. Patent Application Publication 2005/0099373 A1. This method uses a conventional passive color comprised of circularly polarizing selective reflection bands of at least four cholesteric filters together with three liquid crystal switches and related retarder layers. Because this filter only works with polarized light, it is not suitable for cameras used in outdoor and most indoor photography as sunlight and most artificial light sources generate non-polarized light.
Accordingly, it is desirable to provide a new color image sensor system. This system employs a tunable stack of liquid crystal display (LCD) filters which are used to pass a sequence of different colors of light to a sensor array. The system also employs a sensor array optimized to store sequential light information. Furthermore, other desirable features and characteristics of the present disclosure will become apparent from the subsequent detailed description of the disclosure and the appended claims, taken in conjunction with the accompanying drawings and this background.